Electric machines in parallel hybrid drive-trains of motor vehicles can be decoupled from direct restriction to the rotational speed range of the internal combustion engine by the use of an additional step-up or step-down stage, a so-termed high-drive stage, which provides a functional drive arrangement between the driveshaft of an internal combustion engine and the input shaft of a vehicle transmission. Such a high-drive stage enables the use of electric machines with high rotational speeds having a high mass-power density, as can be realized for example in permanently energized synchronous machines. In this way the increasing demand for expensive materials such as rare earth metals and copper, which are needed for the production of modern electric motors, can be reduced, thereby conserving raw material resources and saving manufacturing costs. In addition the installation space and the weight of the electric machine can be reduced. The lower torque of a smaller electric machine can be compensated by a higher rotational speed compared with a rotational speed of a drive input shaft of the vehicle's transmission. The adaptation of the higher rotational speeds of electric machines to a required transmission input rotational speed is effected by virtue of a transmission ratio of the high-drive stage.
From DE 10 2013 211 225 A1 by the present applicant a hybrid drive assembly for a motor vehicle with an electric machine is known, which is in driving connection, by way of a high-drive stage, with a transmission of the vehicle. The vehicle transmission is an automated shift transmission comprising, on the input side, a two-stage, synchronized splitter group, a multi-gear, claw-shifted main transmission with a reversing gear stage, and a two-stage, claw-shifted range group in drive connection downstream therefrom. The splitter group and the main transmission are of countershaft design, whereas the range group is a planetary structure. The electric machine is a permanently energized synchronous machine arranged on the transmission input side. It comprises a stator and rotor and is designed for relatively high rotational speeds, which are adapted to lower transmission input rotational speeds by means of the high-drive stage. The high-drive stage is in the form of a planetary gearset comprising a central sun gear, an outer ring gear and a planetary carrier. On the planetary carrier, a number of planetary gearwheels are mounted to rotate, these meshing with the ring gear and the sun gear between which they rotate. The sun gear is drive-connected to the rotor of the electric machine, whereas the planetary carrier is connected in a rotationally fixed manner to a transmission input shaft and can be drive-connected on the drive input side to an internal combustion engine by means of a starting or separator clutch. The ring gear is permanently fixed to a housing. Between the rotor of the electric machine and the transmission input shaft, by virtue of the high-drive stage, there is a rotational gear ratio such that the rotational speed of the electric machine, when the latter is operating as a motor, is stepped down to a slower speed at the transmission input. If the vehicle is in overdrive operation and if therefore the electric machine is being operated as a generator, then the transmission input rotational speed is stepped up to a higher value at the electric machine. The vehicle transmission also has an oil pump driven by a transmission output shaft, and this supplies lubricating and cooling oil both to the vehicle transmission and to the electric machine and the high-drive stage.
Furthermore, from DE 10 2014 209 056 A1 by the present applicant a hybrid drive assembly is known, which comprises a main transmission with a transmission input shaft and a transmission housing, an electric machine that comprises a stator and a rotor, which can be operated as a motor and a generator and which is arranged on the input side of the main transmission, and a planetary gearset arranged between the rotor and the transmission input shaft, wherein the transmission input shaft has a central oil bore and transverse bores for the supply of oil to the planetary gearset and the electric machine, wherein the planetary gearset and the electric machine are arranged in a hybrid housing, and wherein the hybrid housing is connected to the transmission housing of the main transmission by way of an oil return pipe.
A relatively small and fast-rotating electric machine in combination with a high-drive stage requires increased cooling to carry away the considerable amount of heat that they generate during operation. This can be achieved, for example, by surrounding the stator of the electric machine with a water jacket and in addition, as described in DE 10 2013, 211 225 A1, by supplying lubricating and cooling oil from an oil circuit of the vehicle transmission to the components of the electric machine. However, since only a small and limited fraction of the volume flow of lubricating and cooling oil from an oil pump can be diverted away from the transmission, until now the electric power of the electric machine once a certain temperature threshold has been exceeded is automatically throttled down in order to protect the electric machine and the components around it from overheating and possible damage. This so-termed “derating” of the electric machine can result in a significant temporary reduction of the available electric drive power or generator performance in the drive-train and thereby have an adverse effect on driving comfort and drive power. The incorporation of a larger oil pump, which would constantly deliver a larger volume flow of lubricating and cooling oil, does not seem very appropriate since that would result in higher drag losses in the transmission, it would take up more structural installation space, and it would increase manufacturing costs.